1. Field of the Invention
The present invention relates to a magnetic recording medium, such as a magnetic recording medium which is useful, for example, as a magnetic recording means in an external memory apparatus (a magnetic disk recorder) for computers, etc., and a process for its production.
2. Discussion of Background
A magnetic recording medium is available usually in the form of a tape, a disk or a drum. Among them, a disk type of magnetic recording medium is composed of a rigid substrate of e.g. an aluminum alloy and a magnetic recording layer formed thereon, and further a lubricant is coated on the surface thereof. Coating of such a lubricant is conducted usually by a spin-coating method or a dipping method. It is attempted to improve the abrasion resistance of the magnetic recording medium by coating the lubricant. For example, in the dipping method, a magnetic recording medium is immersed in a solution having the lubricant dissolved in a solvent and then withdrawn, and the solvent is dried to obtain a magnetic recording medium having the lubricant coated thereon. In a drive apparatus for a magnetic recording medium which is presently commonly employed, a contact-start-stop (C.S.S.) system is adopted wherein a head is brought in contact with the magnetic recording medium as the drive apparatus is stopped and driven. Accordingly, the coated lubricant is required to provide durability in C.S.S. test by use of the above drive apparatus. A magnetic recording medium having excellent durability in C.S.S. test is required to have the coated lubricant uniformly deposited on the surface of the magnetic recording medium even after the storage of the magnetic recording medium for a long period of time in a high temperature, low temperature or high humidity environment, or even after the C.S.S. test over a long period of time. Particularly, it is important that the thickness of the lubricant does not change in the above environment or during the C.S.S. test.
As the lubricant to be coated on the magnetic recording medium, it is common to employ a commercially available perfluoropolyether type lubricant. However, a conventional magnetic recording medium using such a lubricant had a problem with respect to the durability in C.S.S. test. Namely, in a high temperature high humidity environment, a migration phenomenon is likely to take place wherein the thickness of the coated lubricant decreases, whereby the C.S.S. durability is substantially impaired. On the other hand, in a low temperature environment or a high humidity environment, the viscosity of the lubricant decreases rapidly, and the lubricating property of the lubricant tends to be inadequate, whereby the durability tends to be impaired.
Further, the process of coating the lubricant used to be complicated including heat treatment of the magnetic recording medium prior to or after the coating and a cooling step after the heating, in addition to the coating of the lubricant.
As the solvent for the above lubricant, a fluorinated solvent containing chlorine atoms in its molecule, such as trichlorotrifluoroethane, is employed. However, such a solvent having a low boiling point has recently be regarded as a factor causing destruction of the environment, and its use will certainly be regulated respectively in the future.